Lasers are commonly used for optical probing of materials and devices because of their ability to provide almost monochromatic optical radiation at a well-defined wavelength with low beam divergence and sufficient intensity. Laser diodes have proven to be very useful in such applications as laser spectroscopy and in testing of optical characteristics of fiber-optic and other optical components. However, a typically very narrow optical spectrum of lasers is associated with a high degree of coherence of the laser radiation, which can lead to undesirable noise-like interference effects during optical test measurements.
Therefore, in some applications of diode lasers it is desirable to effectively reduce the coherence of the laser, to minimize the interference effects. An example is a fiber optic loss meter, where power-calibrated laser diode radiation is transmitted through a fiber-optic component, and the transmitted radiation is detected by a power meter to obtain a power reading. During the measurement, interference effects between fiber connectors can cause variations in the power readings. A known solution to this problem is to effectively reduce the laser coherence by modulating the laser wavelength with a triangle or sinusoidal wave pattern to cause averaging of the interference effect by a light-detecting device used in the measurement. For diode lasers, wavelength modulation can be accomplished by modulating the laser drive current, for example with a triangle wave pattern.
It is known in the art that a diode laser's wavelength varies with the drive current for several reasons, including changing refractive index of the laser diode chip due to changing chip temperature, and changing the refractive index due to injected free carriers. Modulating the laser current therefore has the effect of varying the laser wavelength over time, effectively broadening the laser spectrum and hence reducing the laser coherence, so that a power reading averaged over a time greater than the modulation period will have reduced interference effects.
This method is effective for fiber optic components with fiber pigtails, where the optical path differences (OPD) causing the interference effects are typically greater than 1 meter due to the length of the fiber pigtails. However, in some applications interference effects from surfaces only a few centimeters apart must be minimized. In these applications, simple modulation functions such as a triangle wave or a square wave may not provide satisfactory results.
It is therefore an object of the present invention to provide a method of modulating a drive current of a laser diode using a modulation function optimized for reducing coherence of laser diode radiation.